The present invention is directed to flame retardant compositions comprising an aromatic polyester resin having rheological characteristics suitable for the preparation of foamable cellular materials and the foamed materials therefrom.
Foamed cellular materials (foams) from aromatic polyester resins, representatively polyethylene terephthalate (PET), are having increasingly greater application in the automotive and domestic field either for thermal insulation or as structural material.
For these applications, it is necessary that the foamed material be endowed with good flame retardant properties.
The addition of flame retardant compounds to aromatic polyester resins having rheological properties suitable for the formation of foams endowed with valuable properties has the effect of considerably lowering the rheological properties of the resins and to rendering the same not suitable for the preparation of the foams. Some compounds also cause such remarkable degradations to the resins as to prevent their use as plastic material.
The deterioration of the rheological properties is due to the high temperatures used in the process for the preparation of the foams.
It is strange that the deterioration manifests itself only in resins having high properties of melt strength and viscosity in the melt and not in resins having relatively low melt strength values, and which are not suitable for the preparation of foams with satisfactory properties.
It has been found that flame retardant compounds usable in the compositions of the present invention are those that do not cause a decrease of the rheological properties of the resins corresponding to a decrease of the torque intake lower than 3 Nm in a heating test of the resin added with 10% by weight of the flame retardant compound, operating at 280xc2x0 C. in a rheomix under nitrogen for 10xe2x80x2.
Flame retardant compounds that satisfy the above mentioned test include aromatic brominated compounds containing more than 50% by weight of bromine and having a weight loss at heating at 200xc2x0 C. less than 5% (determined thermogravimetrically with heating speed of 10xc2x0 C./min.).
Representative compounds are N,Nxe2x80x2-ethylene-bis(tetrabromophthalimide), tetrabromophthtalic anhydride, decabromodiphenylether, octabromodiphenylether, bis (tribromophenoxy)ethane, bis(bromophenoxy)ethane.
Of these, the N,Nxe2x80x2-ethylene-bis(tetrabromophthalimide) is the preferred compound thanks to its high melting point, the good stability and the reduced degrading effect on the polyester resin. The compound is commercialized under the trademark Saytex BT 93 of Albermarle. Another bromide compound particularly suitable is commercialized under the trademark Saytex 8010 of Albermarle.
The quantity of the flame retardant compound used is from 1 to 15% by weight.
In the case of tetrabromophthalimide when using 10% by weight of compound, foams classified as B1 are obtained according to the flammability test DIN 4102 and as M2 according to test UNE 23727-90. With 5% by weight of Saytex 8010, foams classified as M1 are obtained according to test UNE 23727-90.
Another usable compound, not comprised in the class of brominated compounds, is basic aluminum oxalate sold by Alcoa under the name BAO.
The foams of the present invention, containing flame retardant agents, have properties comparable with those of the foams without such additives.
The brominated compounds can be used in mixtures with antimony compounds such as sodium antimoniate, antimony trioxide and antimony pentoxide in quantities of 0.5-5% by weight. In the case of sodium antimoniate, the quantity is 0.5-10% by weight; in the oxides case, it is 1-10% by weight.
In the case of sodium antimoniate, it is also possible to use silica in quantity of 0.05-2% by weight of the resin.
The flame retardant compounds can be added before or during the extrusion-foaming step.
They can be added as such or in form of a masterbatch based on polycarbonates, polybutylene terephthalate or polyethylene.
As already indicated, the addition of the above brominated compounds to polyester resins not having the necessary rheological properties, for example a branched PET obtained with trimellitic anhydride, (I.V.=0.95 dl/g) having insufficiently high melt strength or a linear PET of high I.V. (1.1 dl/g) obtained by polycondensation in the solid state without the use of pyromellitic dianhydride and having melt strength less than 8 centiNewton, doesn""t sensibly lower the rheological properties of the polymer.
The torque intake of PET added with 10% by weight of a flame retardant compound such as Saytex BT 93 remains unchanged (test at 280xc2x0 C. in a rheomix for 10 minutes, under nitrogen).
The preparation of the foamed cellular material from the compositions of the present invention is made according to known methods by extrusion-foaming in the presence of a foaming agent. The polyester resins usable for the preparation of the foams of the present invention are characterized by melt strength values greater than 8 centiNewton and by an intrinsic viscosity higher than 0.8 dl/g.
The above mentioned values refer to the resin prior to the extrusion-foaming process as well as to the resin that, under the extrusion conditions, reaches such values during the extrusion step.
A method for obtaining the polyester resins having rheological properties suitable for the formation of the foams of the present invention comprises extruding the resin with I.V. 0.5 -0.7 dl/g added with a dianhydride of a tetracarboxylic acid, preferable an aromatic acid, (preferably pyromellitic dianhydride) in quantities from 0.1 to 1% by weight and subjecting the resin to polyaddition in the solid state until it reaches final viscosity values of 0.85-1.95 dl/g.
The dianhydride of the tetracarboxylic acid can be added to the resin in the form of a masterbatch with polycarbonate, polybutylene terephthalate or with polyethylene in quantities from 1 to 50% in weight.
Another method comprises extruding the polyester resin having intrinsic viscosity from 0.85 to 1.95 dl/g added with a masterbatch of the dianhydride containing polycarbonate, polybutylene terephthalate or polyethylene containing 1-50% by weight of dianhydride.
The masterbatch is added in quantities corresponding to a concentration of dianhydride in the polyester resin from 0.1 to 1% by weight. The dianhydride in the masterbatch is preferably pyromellitic dianhydride.
The aromatic polyester resins usable in the compositions of the present invention are obtained by polycondensation of an aromatic dicarboxylic acid (representatively terephthalic acid or its lower alkyl esters), with an aliphatic diol with 2-10 carbon atoms (representatively ethylene glycol, 1,4-butylene glycol, 1,4-cycloherandimethyol).
The preferred resin is polyethylene terephthalate or copolyethylene terephthalate in which up to 20% of units from terephthalic acid are substituted with units deriving from isophthalic acid or naphtalene dicarboxylic acids.
The intrinsic viscosity is measured in solutions of 0.5 g polyester in 100 ml of a 60/40 mixture by weight of phenol- tetrachloroethane at 25xc2x0 C. operating according to ASTM D 4603-86.
The melt strength is measured by extruding a polymer filament in the capillary of a Goettfert rheometer provided with a capillary having a geometry of 30 mm in length and 1 mm in diameter and with an entrance angle of 90xc2x0.
The piston velocity is 0.2 mm/s. The filament is taken between steel cog-wheels to which a linear acceleration of the peripheral velocity is applied. The acceleration is of 60 mm/s2, and the reported tension is the strength that the filament applies to the wheels at the maximum velocity (1000 mm/s).
The measurement is made at 270xc2x0 C. according to ASTM D 4440.
The measurements of compression set and of the compression module are made according to ASTM D 1621 and D 1623.
The following examples are provided to illustrate and not to limit the present invention.